It is proposed to continue experiments to establish the 5'-terminal sequence of the mRNAs for immunoglobulin L-chains. The procedure involves 1) Purification of intact L-chain mRNA from a number of mouse myeloma cell lines synthesizing immunoglobulins containing kappa-L-chains. 2) Removing the 5'-terminal cap structure with tobacco phosphodiesterase. 3) Removing the exposed 5' phosphate with alkaline phosphatase. 4) Labelling the 5' end with 32P-phosphate using polynucleotide kinase. 5) Isolation of labelled molecules on polyacrylamide gels. 6) Elution of intact material. 7) Partial digestion with sequence-specific or partially sequence-specific enzymes. 8) Resolution of the digestion products on one-dimensional polyacrylamide gels. Comparison of a number of 5'-end sequences from various mRNAs belonging to the kappa-class will be made so as to elucidate the relationship between 5'-terminal signals (e.g., the ribosome binding site) and the V-genes encoded in mouse myeloma cells.